The present invention is directed to a method and an arrangement for encoding and decoding a digitized image with picture elements.
In image compression methods for encoding or decoding digitized images, the images are usually divided into image segments.
A distinction is made between two approaches for image encoding, object-based image encoding and block-based image encoding. Object-based methods for image encoding are described in S. Hofmeir, “Multimedia fur unterwegs”, Funkschau, No. 7, 1996, pp. 75–77. An overview of block-based image encoding methods can be found in D. Le Gall, “MPEG: A Video Compression Standard for Multimedia Applications”, Communications of the ACM, Vol. 34, No. 4, April 1991, pp. 47–58 (MPEG); M. Liou “Overview of the px64 kbits/s Video Coding Standard”, Communications of the ACM, Vol. 34, No. 4, April 1991, pp. 60–63 (H.263); and G. Wallace, “The JPEG Still Picture Compression Standard”, Communications of the ACM, Vol. 34, No. 4, April 1991, pp. 31–44 (JPEG).
Given block-based image encoding methods, the image is divided into what are referred to as image blocks that usually comprise a rectangular shape and respectively comprise 8×8 or 16×16 picture elements. In the known methods, the blocks are transformed with the assistance of a transformation encoding, preferably discrete cosign transformation (DCT), wavelet transformation or a transformation with vector quantization.
Losses in the image quality must be accepted in the transmission of moving images over narrow-band channels such as, for example, 40 kbit/sec or lower for picture telephony applications. The most noticeable disturbances are the brightness discontinuities known as block artifacts in block-based image encoding or as object edge artifacts in object-based image encoding, i.e. the abrupt changes of the values of the encoding information that is allocated to the individual picture elements produced by discontinuity points at the image block edges or at the image object edges.
What is to be understood below by encoding information is, for example, luminance information or chromance information that is respectively unambiguously allocated to the picture elements.
Two different approaches are known in order to reduce the block artifacts.
The first approach is based on corrections in the frequency domain of the spectral transformation. A method referred to as spatial shaping reduces the edge artifacts at the expense of the image quality in the interior of the block. This method is known from W. Gerod et al., “Spatial Shaping: A Fully Compatible Improvement of DCT-Coding”, Picture Coding Symposium, Lausanne, France, 1993. Another method that is based on corrections in the frequency domain employs the prediction of the DCT coefficients. Although the quality in the interior of the block is improved by this procedure described in R. Kutka, A. Kaup and M. Hager, “Quality Improvement of Low Data-Rate Compressed Signals by Pre- and Postprocessing”, Digital Compression Technologies and Systems for Video Communications, SPIE, Vol. 2952, 07–09 Oct. 1996, pp. 42–49, the block artifacts are only partially reduced.
The second approach for reducing block artifacts is based on corrections in the location domain. S. Minami and A. Zakhor, “An Optimization Approach for Removing Blocking Effects in Transform Coding”, IEEE Transactions on Circuit Systems Video Technology, Vol. 5, No. 2, April 1995, pp. 74–82 discloses that the picture elements at the block edges be subjected to a low-pass filtering, as a result whereof the discontinuity points are smoothed and appear less disturbing. H.245 Standard, ITU Standard Recommendation discloses that different filters be employed for different image blocks, dependent, for example, on the quantization of the image block or dependent on the motion vector.
German Patent Application No. 196 040 50 discloses a method for controlling various transmission parameters in the framework of the H.263 standard, which is referred to as J.245 standard. It is known within the framework of the H.245 standard to inform a second arrangement with which communication is desired of specific transmission properties from a first arrangement via what is referred to as a capability table wherein the respective feature that is to be employed within the framework of the communication connection is indicated.